I. Field of the Invention
The present invention deals generally with liquid heating systems typified by water heating systems. More particularly, the invention involves a clean out alerting system for scale buildup in water heating systems which derives from changes in recovery heating efficiencies in such systems. A decrease in efficiency, it has been learned, may be noted based on a percentage increase in the average time required to heat the water from the temperature at which the system calls for heat to the set point temperature, i.e., the duration of the ON portion of the cycle.
II. Related Art
Hot water tanks, boilers and the like have long provided sources of commercial hot water for a variety of purposes. These devices may be tankless but typically include a vessel for containing a volume of water to be heated and contained within a metal outer tank structure. Heating may be electrical, using one or more heating elements geometrically arranged and immersed within the volume of water, or gas heated, including a burner system and one or more heat exchangers. The tank or similar device is suitably attached to a source of make up water and one or more external devices for using the heated water such as faucets, radiators or other heat exchangers or the like.
The systems are thermostatically controlled about a manually adjustable set point calling for heat when the sensed water temperature falls a preset amount below the set point temperature and shutting off the energy input when the set point temperature is regained. This sequence is known as a heating cycle and is repeated many thousands of times over the life of the heating vessel.
Regardless of the type of heating unit involved, tank, tankless, boiler, etc., mineral deposits called scale form during the water heating process. These deposits form on the hot heat exchanger surfaces of the unit and create an insulating layer which builds and reduces heat transfer efficiency or decreases dissipation of input energy which also causes the temperature of the outside metal surface to increase. Continued buildup further reduces heat transfer and further increases the metal heat level. In this manner operating costs increase due to the lower heat transfer efficiency and the life of the heating unit decreases due to overheating. In some high duty applications which require large amounts of hot water such as restaurants, laundries, hotels and motels, etc., water heaters develop deposits quickly, causing short product life and thus frequent heater replacement.
The problem of scale buildup has been traditionally addressed by either of two approaches, i.e., by carrying out periodic cleaning on a regular basis or by adding water treatment equipment to the system. The first approach probably will not mimic or reflect properly the actual cleaning needs of the system and relies on guesswork. If the time between cleanings is too short, cleaning will be undertaken too often and thus not be cost effective. If the interval is too long, appliance life and efficiency are again sacrificed. The time variable nature of water use also works to thwart the desirability of this approach. The second solution is even more impractical for all but the largest industrial applications owing to the high cost of water treatment solutions.
In the past commercial water heater controls were rather unsophisticated ON-OFF electromechanical devices that turned a burner or other energy source on when a thermostat called for heat following a drop in temperature and turned the energy source off when the water temperature reached the set point temperature. More recently, the introduction of microprocessor based electronically controlled technology has enabled the sophistication of such control systems to be greatly expanded. This includes the sensing and the integrating of information pertaining to additional operating characteristics. It would be desirable if this potential could be harnessed to provide a more accurate estimate of the amount of accumulated scale in a water heater, boiler, or other such vessel.
It is known from U.S. Pat. No. 4,445,638 to measure the rate of rise of the temperature of boiler water to identify the existence of a low water condition perceived as an abnormally rapid rate of water temperature rise. This information is used to insure that the system operates in a proper rust-inhibiting mode and can be used to shut the system down if a preset minimum heating or recovery time limit is not reached. It is further known to incorporate a microprocessor in water heater control systems for a variety of reasons, for example, U.S. Pat. No. 5,797,358 depicts microprocessor control of temperature set point programming and burner control that prevents operation in the presence of detected unsafe conditions.